Abstract

AbstractUnintentional mortality of higher trophic‐level species in commercial fisheries (bycatch) represents a major conservation concern as it may influence the long‐term persistence of populations. An increasingly common strategy to mitigate bycatch of harbor porpoises (Phocoena phocoena), a small and protected marine top predator, involves the use of pingers (acoustic alarms that emit underwater noise) and time‐area fishing closures. Although these mitigation measures can reduce harbor porpoise bycatch in gillnet fisheries considerably, inference about the long‐term population‐level consequences is currently lacking. We developed a spatially explicit individual‐based simulation model (IBM) with the aim to evaluate the effectiveness of these two bycatch mitigation measures. We quantified both the direct positive effects (i.e., reduced bycatch) and any indirect negative effects (i.e., reduced foraging efficiency) on the population size using the inner Danish waters as a biological system. The model incorporated empirical data on gillnet fishing effort and noise avoidance behavior by free‐ranging harbor porpoises exposed to randomized high‐frequency (20‐ to 160‐kHz) pinger signals. The IBM simulations revealed a synergistic relationship between the implementation of time‐area fishing closures and pinger deployment. Time‐area fishing closures reduced bycatch rates substantially but not completely. In contrast, widespread pinger deployment resulted in total mitigation of bycatch but frequent and recurrent noise avoidance behavior in high‐quality foraging habitat negatively affected individual survival and the total population size. When both bycatch mitigation measures were implemented simultaneously, the negative impact of pinger noise‐induced sub‐lethal behavioral effects on the population was largely eliminated with a positive effect on the population size that was larger than when the mitigation measures were used independently. Our study highlights that conservationists and policy makers need to consider and balance both the direct and indirect effects of harbor porpoise bycatch mitigation measures before enforcing their widespread implementation. Individual‐based simulation models, such as the one presented here, offer an efficient and dynamic framework to evaluate the impact of human activities on the long‐term survival of marine populations and can serve as a basis to design adaptive management strategies that satisfy both ecological and socioeconomic demands on marine ecosystems.

Highlights

  • A major concern in the successful conservation of marine systems is fisheries bycatch (Kappel 2005, Komoroske and Lewison 2015)

  • We developed a predictive modeling tool that can assess the population-level consequences of harbor porpoise bycatch mitigation using time-area fishing closures and pingers that emit randomized highfrequency signals

  • Our study showed that the joint implementation of time-area fishing closures and pingers produced a population size that exceeded those in model scenarios where bycatch mitigation measures were used independently

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Summary

Introduction

A major concern in the successful conservation of marine systems is fisheries bycatch (Kappel 2005, Komoroske and Lewison 2015). There, the implementation of time-area fishing closures and thereafter the use of pingers on gillnets coincided with a reduction in bycatch of harbor porpoises from ~2900 individuals/yr in 1990 to 323 individuals/yr in 1999 (Read 2013). This trend is generally considered a conservation success story. It remains unclear whether the mitigation measures were the main cause of the reduction in harbor porpoise bycatch or whether the trend was confounded by effort controls in the fishery industry itself (Geijer and Read 2013). Data on changes in the harbor porpoise population size during the study period were scant, inhibiting conclusions about the population-level consequences of reduced bycatch rates and employed mitigation measures (General Accountability Office 2008)

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